1. Field of the Invention
The present invention is related to a focusing device that includes a differential interference prism used in differential interference observation in a focusing detection optical system.
2. Description of the Related Art
Conventionally, a microscope device has been used that includes a focusing detection mechanism (e.g., an active AF (Auto Focus) mechanism) and a differential interference observation mechanism. In such a microscope device, a portion of a focusing detection optical system is used in common with a portion of a differential interference observation optical system, and a differential interference prism is provided to the portion of the focusing detection optical system. Therefore, also in the focusing detection by the focusing detection mechanism in differential interference observation, light (in this case, light for focusing detection) passes through the differential interference prism.
As an example, in a focusing device described in Japanese Laid-Open Patent Publication No. 2000-199846, when it is determined that focusing is not performed, a differential interference prism is driven outside an optical path, and a focusing determination is performed. After focusing is performed, the differential interference prism is driven to return in the optical path with a focusing position maintained so as to enable focusing detection.
In recent years, not only enabling highly accurate focusing detection but also shortening the time needed for the focusing detection has been desired strongly.